Winter and summer

In January 1996 a man in the north-eastern United States went out after a blizzard to check that his car would still start. He made, in the words of the report the CDC published about that storm, "no attempt to remove any snow surrounding the car before starting the engine."¹ He was found unconscious in his car about an hour later. He lived, and when the report was written his short-term memory was still affected.

Summer has its own version: a car standing still, an adult who forgot the child was there or meant to be back in a minute, and a child inside. NHTSA, the US federal road safety regulator, gives the figure for 2025: 31 children dead of heatstroke in vehicles.²

Neither of those cars was being driven. This lesson is about the two seasons in which a parked car can kill the people sitting in it, and about the duller work that heads off some seasonal breakdowns: getting the car ready for the cold and the heat, and clearing it before you drive.

A breakdown in snow or heat is still a breakdown, and the callout's steps and lesson 16, "When the car stops: where, who stands where, and the call", are what you follow when it happens. One section below states the law, on clearing a car in Great Britain. This is general information, not legal advice. The rules differ between England and Wales, Scotland, Northern Ireland and each US state, and they change.

The lesson uses UK words with the US word once: windscreen (windshield), bonnet (hood), tyre (tire), exhaust pipe (tailpipe). Temperatures are given in °C with °F, and the US sources' own figures are in °F with the course's conversion beside them.

Getting the car ready for winter

Four lists cover the same ground: two from the Highway Code, one from NHTSA, and the RAC's, which the Met Office page reproduces (the RAC sells breakdown cover and repairs). It helps to see them side by side before deciding what's yours to do.

Most of what these lists ask for, you've already met as a check of its own. What winter adds is a reason to do them before the first cold spell rather than after it.

The battery. The Met Office page, reproducing the RAC's advice, says "Battery problems are the reason for more RAC call-outs in winter than anything else."⁴ That's the RAC's count of its own call-outs, and the RAC sells breakdown cover and repairs. NHTSA's version is a single line, "When the temperature drops, so does battery power", and its advice is to "Have a mechanic check your battery, charging system, belts, and for any other needed repairs or replacements."⁵ Having the battery tested is Call someone: a mechanic, in NHTSA's words. Lesson 5, "The 12 V battery: what it does, and why short trips flatten it", explained why winter is when a battery fails and not usually when it was damaged.

Screenwash and coolant. Lesson 9, "Under the bonnet: oil, coolant, brake fluid and screenwash", covered both: the RAC's screenwash line on the Met Office page, "effective down to at least -15 degrees Celsius", the handbook's coolant, and never a hot cap. NHTSA says the same for the US in its own words: "Make sure your vehicle's reservoir is full of high-quality "winter" fluid with de-icer before winter weather hits."⁵ Topping up screenwash is Yours; topping up coolant is Yours, on conditions, cold and with the handbook's coolant, as lesson 9 set out.

Tyre pressure. NHTSA: "As the outside temperature drops, so does tire inflation pressure."⁵ Lesson 7, "Tyres I: pressure, the placard and TPMS", said that a tyre set right in the autumn can read low in midwinter without a leak. Winter is when that shows, so a cold check with a gauge against the placard, which is Yours, belongs on the list.

Wipers and floor mats. Lesson 10 took the wipers. NHTSA adds one item that isn't on the UK lists: thicker winter mats, which, NHTSA warns, are dangerous if fitted wrongly: "Improperly installed floor mats in your vehicle could interfere with the operation of the accelerator or brake pedal, increasing the risk of a crash."⁵ Its answer is the maker's instructions and the retention clips.

Electric cars. NHTSA explains that a battery uses its own charge to warm itself in the cold: "The battery drain due to heating can be minimized by keeping your electric vehicle as warm as possible during freezing temperatures." And then: "A common way to do this is plugging your vehicle in at night during the winter".⁵ Lesson 4, "Hybrids and electric cars: how they work", gave the cold-weather range figures.

Fuel economy in the cold. Lesson 12, "Fuel economy: what maintenance buys, and the figures that were withdrawn", promised this. fueleconomy.gov, run by the US Department of Energy and the Environmental Protection Agency, says that "Fuel economy tests show that, in city driving, a conventional gasoline car's gas mileage is roughly 15% lower at 20°F than it would be at 77°F. It can drop as much as 24% for short (3- to 4-mile) trips."⁷ That's about −7 °C against 25 °C, by this course's conversion. Here the source does give a mechanism: "Engine and transmission friction increases in cold temperatures due to cold engine oil and other drive-line fluids", and "It takes longer for your engine to reach its most fuel-efficient temperature."⁷ Its view of warming the car up before you set off fits in one line: "idling gets 0 miles per gallon".⁷

Winter tyres

In the UK, the RAC, which sells breakdown cover and repairs, says "Winter tyres aren't mandatory in the UK."⁸ The Met Office page, reproducing the RAC, recommends "ideally a minimum tread depth of 3mm in icy or very wet conditions".⁴ That's the RAC's recommendation, above the legal floor lesson 8, "Tyres II: tread, damage, age, and when to replace", set out. In the US, NHTSA's line is "Consider installing snow tires".⁵ The RAC's line is about the UK; NHTSA presents snow tyres as something to consider, and this course read no state's rule on them. Whether your car can take snow chains is your handbook's to say, and where they're required is the road authority's; this course doesn't teach them.

Winter law: clearing the car in Great Britain

This is general information, not legal advice.

The Highway Code applies to England, Scotland and Wales, and its Introduction sets the convention: rules with "MUST/MUST NOT" are legal requirements, and it names the law behind each.⁹ Everything else is advice, though the Code "may be used in evidence in any court proceedings under the Traffic Acts", and that "includes rules which use advisory wording such as 'should/should not' or 'do/do not'."⁹ Northern Ireland has its own Highway Code, which this course did not read.

Rule 229, under the heading "Before you set off", says:⁶

• "you MUST be able to see, so clear all snow and ice from all your windows"
• "you MUST ensure that lights are clean and number plates are clearly visible and legible"
• "make sure the mirrors are clear and the windows are demisted thoroughly"
• "remove all snow that might fall off into the path of other road users"
• "check your planned route is clear of delays and that no further snowfalls or severe weather are predicted."

The first two are law. The rule cites "CUR reg 30, RVLR reg 23, VERA sect 43 & RV(DRM)R reg 11",⁶ and lesson 10, "Lights, wipers, glass and the walk-round", quoted regulation 30 of the Construction and Use Regulations: fitted glass is to be kept so that it "does not obscure the vision of the driver". Notice what the MUST says: all snow and ice, from all your windows. A clear patch in front of your face does not meet it. The last three lines are advice, including the snow on the roof. This course read no law on snow left on a roof, and makes no statement about one.

Clearing the car is Yours, and it takes time, which is where the second rule comes in. Lesson 12 quoted Rule 123 in full. Its MUST line: "You MUST NOT leave a parked vehicle unattended with the engine running or leave a vehicle engine running unnecessarily while that vehicle is stationary on a public road."[10] On this course's reading, leaving the car on the street with the engine running to defrost while you go back inside is what the first part of the rule forbids. The rule's words are about a public road; this course read nothing on a private drive, and does not say what applies there. Annex 6's advice on security points the same way wherever the car is: when you leave it, "remove the ignition key and engage the steering lock" and "lock the car, even if you only leave it for a few minutes".³

One more thing about running an engine while you clear a car. Never do it inside a garage, even with the door open. The CDC, the US public health agency, says: "Never run your car or truck inside a garage that is attached to a house, even with the garage door open."[11] That sentence is about an attached garage, and the same page tells readers to open a detached garage's door; keeping a running engine out of every garage is this course's own precaution, stricter than the CDC's. Carbon monoxide in the house, and a car in an attached garage, are Home Repair and Maintenance lesson 11's, "Carbon monoxide and the alarms that warn you".

A frosted car in Inverurie

It's 7 a.m. in Inverurie, in Scotland, where the Highway Code applies. The car has been out all night on the street, and every window is white. The driver has twenty minutes.

Step 1: the engine. The tempting move is to start the car, turn the heater to full and go back in for a coffee. On this course's reading, Rule 123's first part covers that: a parked car, unattended, engine running, on a public road. Staying with the car answers only that first part, on the same reading. The regulation behind it, regulation 107 of the Construction and Use Regulations, which lesson 12 described, speaks of a car "not attended by a person licensed to drive it", and this course read nothing on what counts as attending. The second part is about running an engine "unnecessarily" while stationary on a public road. The Code names only standing in traffic and diagnosing faults as times it's permissible, and this course read nothing that says whether defrosting is one. The rule's advice, its should line, is clear, though: "Generally, if the vehicle is stationary and is likely to remain so for more than a couple of minutes, you should apply the parking brake and switch off the engine".[10] So how the law's second part applies to defrosting is a question this course can't answer, and the Code's advice is not in doubt. Twenty minutes of standing is a switched-off engine and a scraper.

Step 2: the glass. Rule 229's first MUST is all snow and ice from all windows: the windscreen, the side windows and the rear window, not a peephole. A scraper and de-icer are on Rule 228's kit list.

Step 3: lights and plates. The second MUST: lights clean, plates "clearly visible and legible". Frost on a headlamp or a number plate counts.

Step 4: the rest. Mirrors clear, windows demisted, snow off the roof and bonnet where it could slide onto the windscreen or onto someone else. These are advice. On this course's reading, snow sliding off the roof onto the windscreen at the first junction puts the driver back where Step 2 started.

Step 5: the exhaust. Before the engine runs at all in deep snow, the pipe is clear. The next section explains why.

A car stuck in snow: carbon monoxide

A petrol or diesel engine's exhaust carries carbon monoxide. Toyota's handbook for the 2023 Corolla sold in the US puts it in one line: "Exhaust gases include harmful carbon monoxide (CO), which is colorless and odorless."[13] Snow is what brings it inside.

The mechanism. The CDC's report on the January 1996 blizzard gives it: "CO can seep into the cabin of a vehicle through leaks or cracks in the floorboard if the exhaust pipe is obstructed."¹ The CDC's own advice page puts it for the public: "If the tailpipe is blocked, carbon monoxide, an odorless, colorless, and deadly gas produced by your engine can build up quickly inside your vehicle, poisoning anyone inside."[14] And Toyota describes the other route, snow piled round the car rather than in the pipe: "If snowbanks build up around the vehicle while the engine is running, exhaust gases may collect and enter the vehicle."[13] You can't smell it. The report says CO is "colorless, tasteless, odorless, and nonirritating", so "its presence usually is not detected."¹

What it did in 1996. The same report describes one city's two days after the storm: "a total of 21 persons were admitted directly to the Jacobi Medical Center or transferred from other hospitals for hyperbaric oxygen therapy because of CO poisoning", and "In addition, one fatal case of CO poisoning was reported in which the victim was not admitted to the hospital." All 21 "were found unconscious inside automobiles with engines running." Eight were under 16 and twelve were over 50.¹ Those are one city's figures from one storm, thirty years ago, and they're given here for what they show about how it happens, not as a rate. This course found no UK official page on snow-blocked exhausts in a stranded car, so the sources in this section are American, and one is a maker's handbook for a US car.

One of the report's cases is worth reading slowly. A family was digging their car out of a snowbank, and a four-year-old girl got cold. "To warm the child, she was placed in the car with both the engine and heater running while deep snow surrounded the rear of the automobile." Her grandmother and sister sat in the car too, and got out after a few minutes "when the grandmother became dizzy and lightheaded." The girl, thought to be napping, stayed about 30 minutes "before she was found to be unconscious and could not be awakened." She was treated and discharged.¹ The family was trying to warm a cold child.

What the report asks of everyone is short: "Following heavy snowfalls, the public should be reminded to inspect vehicles to ensure that exhaust pipes are cleared of snow before engines are started."¹ Clearing snow from the exhaust pipe before starting is Yours. A leak in the exhaust itself is not. Toyota's handbook for the Corolla: "If you smell exhaust gases in the vehicle even when the trunk lid is closed, open the windows and have the vehicle inspected at your Toyota dealer as soon as possible."[13] On this course's reading, that makes exhaust fumes inside any car Call someone: a garage.

What carbon monoxide does to a person, and what to do for someone it has affected, is First Aid and CPR's: lesson 14, "Seizures, low blood sugar and poisoning", covers carbon monoxide, and lesson 15, "Heat, cold, bites and stings", covers cold.

When you're stranded in snow

These are US steps, for a car stuck in snow. In Great Britain, a car stopped on a motorway or other fast road follows the callout's steps whatever the weather, and lesson 17, "Motorways and fast roads: go left, the live lane, and the smart motorway question", teaches them in full: out on the side away from traffic, behind the barrier if there is one, and, in the words of the Highway Code's Rule 277, which is advice, "DO NOT return to your vehicle even if it's raining, cold or dark".[18] This course read no UK official advice for a car stuck in snow anywhere else.

NHTSA gives the steps for a car that is "stopped or stalled in wintry weather":⁵

• "Stay with your car and don't overexert yourself."
• "Let your car be seen. Put bright markers on the antenna or windows and keep the interior dome light on."
• "Be mindful of carbon monoxide poisoning. Make sure your exhaust pipe is clear of any snow and run your car only sporadically", just long enough, the page goes on, to stay warm. "Don't run your car for long periods of time with the windows up or in an enclosed space."

Caltrans says much the same for California: "When stalled, stay with your vehicle and try to conserve fuel while maintaining warmth. Be alert to any possible exhaust or monoxide problems."[12]

Toyota's handbook goes further. For the 2023 Corolla sold in the US, under "When parking": "Do not leave the engine running in an area with snow build-up, or where it is snowing."[13] That's stricter than NHTSA, which allows short spells with the exhaust clear. The two agree on the dangerous part, a blocked exhaust and long running, and differ on whether to run the engine at all while it is snowing. On this course's reading, the handbook governs its own car here too, as it does for every figure (lesson 1): if yours says don't, don't. If it says nothing, NHTSA's short spells with the pipe checked each time are the advice this course read, for the US.

NHTSA's kit list for the car is the practical side of staying put: "a snow shovel, broom, and ice scraper;" "abrasive material (sand or kitty litter), in case your vehicle gets stuck in the snow;" "blankets for protection from the cold; and" "a cell phone and charger, water, food, and any necessary medicine."⁵ National Highways, which runs England's motorways and major A roads, gives a similar list for its roads, and its first advice is to "Check the weather forecast. If snow or ice is expected, consider delaying your journey until it clears."[15]

A night near Binghamton

Now one with a gap for you. A car slides into a snowbank on a rural road near Binghamton at 10 p.m. The engine runs, but the car won't move, the snow is still falling, and the rear of the car is buried to the bumper. There are two adults inside. The phone has signal, and help says it can't come until the roads are cleared in the morning.

Getting the car ready for summer

The summer lists are shorter.

The cooling system. National Highways: "During particularly hot weather, make sure your engine stays within the normal operating temperature range. If it begins to overheat, find somewhere safe to stop and allow it cool down."[16] (The missing "to" is the page's.) "Somewhere safe" is lesson 16's subject. Annex 6 adds the rule you met in lesson 9: "If your engine overheats, you should wait until it has cooled naturally."³ A hot cap is never opened, and the temperature light is one of lesson 11's stop-now group.

The people. The Highway Code's Rule 237 says, as advice: "Keep your vehicle well ventilated to avoid drowsiness."⁶ National Highways' summer kit starts with "drinking water", and it asks you to take enough "for you and your passengers".[16]

A child or an animal in a hot car

NHTSA's campaign page puts the scale in two sentences. "More than 1,000 children have died of heatstroke, because they were left or became trapped in a hot car over the past 25 years." And: "In 2025, 31 children died of heatstroke in vehicles."² Those are US figures. This course found no UK figure, and it doesn't borrow the American one for the UK.

Why a child. NHTSA gives one reason: "A child's body temperature rises three to five times faster than an adult's."² That is the whole of the mechanism the sources read here give. None of them explains why a parked car heats up as it does, and this course doesn't supply an explanation of its own. A 2022 heat-wave news release from AAA's Auto Club Group (AAA clubs sell roadside assistance and run a repair network) gives a size, for the US heat it was writing about: "Outside air temperatures in the nineties can rise to 125 degrees inside the vehicle very quickly".[17] In °C that's a day of about 32 to 37 °C becoming about 52 °C inside, on this course's conversion.

How it happens

NHTSA's two figures point at two different ways, and they need two different habits.

The first is forgetting. "Over 50% of pediatric vehicular heatstroke deaths are a result of a parent or caregiver forgetting a child in a car."² NHTSA's answers are habits: "Make it a habit to check your entire vehicle", especially the back seat, "before locking the doors and walking away"; "Place a personal item like a purse or briefcase in the back seat, as another reminder to look before you lock"; and "Ask your childcare provider to call if your child doesn't show up for care as expected."²

The second is children getting in by themselves. "Over 230 children have died from vehicular heatstroke since 1998 because they gained access to a vehicle and became trapped." The answer is the car, not the parent: "Always lock your car when you aren't using it", and "Store car keys and fobs out of a child's reach and teach children that a vehicle is not a play area."² NHTSA addresses the first of these to everyone, children or not, because the child who climbs in might be a neighbour's.

The Highway Code's version is one line in Annex 6's advice on security, under "When you leave your vehicle you should": "never leave children or pets in an unventilated car".³ It's a should, advice. Its word is "unventilated". It doesn't say an open window makes a car safe, and NHTSA says rolling windows down "does little" to the temperature inside. AAA, whose clubs sell roadside assistance and run a repair network, has a line that covers animals too: "Never leave children or animals unattended in a car, not even for a short period of time."[17] This course did not read any law, in Great Britain, Northern Ireland or any US state, on leaving a child or an animal in a car, or on breaking into a car to reach one, and it makes no statement about either. The habits above are Yours, and they're what the sources give.

If you find one

NHTSA, for the US: "If you see a child alone in a locked car, act immediately and call 911. A child in distress due to heat should be removed from the vehicle as quickly as possible and rapidly cooled."² NHTSA doesn't say who removes the child or how, and, as above, this course read no law on breaking into a car and makes no statement about it. This course read no UK page that sets out a bystander's steps for a child in a hot car. In the UK the emergency number is 999. What happens after the child is out, the cooling and the signs of heatstroke, is First Aid and CPR lesson 15, "Heat, cold, bites and stings", and it's worth reading before a summer, not during one.

A five-minute errand in Temecula

A parent in Temecula has a toddler asleep in the back and a prescription to collect. The pharmacy is across the car park. The plan: park under a tree, windows down four inches, back in five minutes.

What people get wrong

"My battery was fine all summer, so the frost killed it." The frost is when a tired battery gives out, not usually what tired it. AAA's Mountain West club, which sells roadside assistance and runs a repair network, puts most of the damage on hot weather, which is why the autumn test in the checkpoint above comes before the cold rather than after it.

"A window open makes a hot car safe." NHTSA: rolling windows down or parking in the shade "does little to change the interior temperature of the vehicle."

"Idling in the snow to keep warm is fine." Only with the exhaust clear and only in short spells, on NHTSA's advice for the US, and not at all if your handbook says otherwise. The CDC's report is about people who did exactly that with the pipe blocked.

"A peephole in the ice will do." In Great Britain, Rule 229's MUST covers all your windows. The section on winter law has the words.

"Winter tyres are required in the UK." The RAC, which sells breakdown cover and repairs: "Winter tyres aren't mandatory in the UK."

Practice

For your plan

Add a seasonal page to your car file: the two lists from the exercise, each item labelled; the handbook's own words on running the engine in snow or an enclosed space; where the winter kit lives in the car; and the two hot-car habits. Lesson 16, "When the car stops: where, who stands where, and the call", adds the breakdown plan that sits next to it.

Connections

Lesson 5 explained why batteries fail in winter and were damaged in summer. Lesson 7 is the cold pressure check, lesson 9 the screenwash and the cold coolant, lesson 10 the glass and lights the law wants clean, and lesson 12 the Rule 123 this lesson applied to defrosting. First Aid and CPR lessons 14 and 15 take over where a person is affected by carbon monoxide, heat or cold. Home Repair and Maintenance lesson 11 covers carbon monoxide in the house and an attached garage. Lesson 16 is next: what to do when the car stops, in any weather.

Go deeper

• Your own handbook, its sections on exhaust gas and cold weather. Toyota's for the 2023 Corolla puts its carbon monoxide warning on page 46, in the safety chapter at the front.
• CDC, "Carbon Monoxide Poisonings Associated with Snow-Obstructed Vehicle Exhaust Systems", MMWR, 1996. Read here in full. Two pages; the three case reports teach more than any list of rules.
• NHTSA, "Heatstroke", free. Read here in full, from an archive capture. The US campaign page, with the full list of habits.
• NHTSA, "Winter Weather Driving Tips", free. Read here in full, from an archive capture. The US regulator's winter checklist for drivers.

Sources

1. Centers for Disease Control and Prevention, "Carbon Monoxide Poisonings Associated with Snow-Obstructed Vehicle Exhaust Systems -- Philadelphia and New York City, January 1996", Morbidity and Mortality Weekly Report, 1996. US. Read from Internet Archive capture 20260725225621 (cdc.gov refuses automated readers; the live page is linked). Read: full text, re-read 26 September 2026.
2. NHTSA, "Heatstroke" (the page is titled "Child Heatstroke Prevention: Prevent Hot Car Deaths"). US. Read from Internet Archive capture 20260924021947 (nhtsa.gov refuses automated readers; the live page is linked). Read: full text, 26 September 2026.
3. The Highway Code, "Annex 6. Vehicle maintenance, safety and security", updated 29 January 2022. Great Britain. Read: full text, re-read 26 September 2026 through the GOV.UK content API. "Before winter", "Vehicle security" and the overheating line are advice, not MUST rules.
4. Met Office, "6 simple checks to prepare your vehicle for winter". The UK's national weather service, reproducing the RAC's checklist (the RAC is a motoring organisation that sells breakdown cover and repairs). No date shown. Read: full text, re-read 26 September 2026.
5. NHTSA, "Winter Weather Driving Tips". US. Read from Internet Archive capture 20260812080428, the newest there is (the live page is linked). Read: full text, re-read 26 September 2026.
6. The Highway Code, "Driving in adverse weather conditions (226 to 237)", updated 14 September 2021. Great Britain. Read: full text, re-read 26 September 2026 through the GOV.UK content API.
7. fueleconomy.gov (US Department of Energy and EPA), "Fuel Economy in Cold Weather". Read: full body text, re-read 26 September 2026.
8. RAC (a motoring organisation that sells breakdown cover and repairs; not an official body), "Winter tyres", dated 2 September
   2025. Read: full text, re-read 26 September 2026.
9. The Highway Code, "Introduction", updated 1 July 2022. Great Britain. Read: full text, 26 September 2026, through the GOV.UK content API.
10. The Highway Code, "General rules, techniques and advice for all drivers and riders (103 to 158)", Rule 123, updated 17 September 2023. Great Britain. Read: full text of the page, re-read 26 September 2026.
11. Centers for Disease Control and Prevention, "Carbon Monoxide Poisoning Basics", 12 January 2026, read from Internet Archive capture 20260920191925 (the live page refuses automated readers). US. Read for this lesson: the page's car and truck prevention lines, 26 September 2026. The live page is linked for the reader.
12. Caltrans (California Department of Transportation), "Winter Driving Tips". California. No date shown. Read: full text, re-read 26 September 2026. Used here for its stalled-vehicle advice only.
13. Toyota Motor Sales USA, 2023 Corolla Owner's Manual (OM02568U), US market, PDF. Read for this lesson: p. 46 ("Exhaust gas precautions"), re-read 26 September 2026. A maker's handbook for its own car.
14. Centers for Disease Control and Prevention, "Clear Snow from Tailpipes", public service announcement toolkit, 7 February 2024. US. Read from Internet Archive capture 20260725201242 (the live page is linked). Read: full text, re-read 26 September 2026.
15. National Highways, "Travelling in snow and ice". The government-owned company that runs England's motorways and major A roads. No date shown. Read: full text, re-read 26 September 2026.
16. National Highways, "Travelling in extreme heat". No date shown. Read: full text, re-read 26 September 2026.
17. AAA (The Auto Club Group, a regional US motoring club that sells roadside assistance; not an official body), "AAA: Hot Tips For Motorists During Extreme Heat", news release, 14 June 2022. Read: full text, re-read 26 September 2026.
18. The Highway Code, "Breakdowns and incidents (275 to 287)", Rule 277, updated 14 September 2021. Great Britain. Read: full text of the page, re-read 26 September 2026 through the GOV.UK content API. Rule 277's "DO NOT" lines are advice, not MUST rules.